Thermowells, chimneys and stacks all represent the physical problem of a blunt body in crossflow. Depending on the diameter of the body and the velocity of the flow, they can be subject to the Von-Karman street shedding phenomenon, leading to Vortex-induced Vibrations (VIV). Oscillating forces induced by the vortex shedding limit the fatigue life of the items and in some cases can lead to resonance failure.
FE Consultants (FEC) were asked to design a Thermowell for a transfer line in an Ammonia plant subject to 815°C mixed hydrogen gas flow at 70ms-1. The line was fabricated, heat-treated and refractory cast. The geometrical limitations of the thermowell were dictated meaning a significant length and limited diameter, resulting in a low natural frequency. Compliance with ASME PTC 19.3 was not possible given the constraints. The thermowell natural frequency could be shifted to an acceptable range with the use of a vibration collar. However, the high temperature of the line made the fit, tolerancing and expansion characteristic challenging. Several instrument vendors were approached, with all proving unsuitable due to technical limitations or unsuitable lead time.
FEC worked with the client and determined a feasible solution using a custom machined helical strake thermowell design. Helical strakes are widely referenced in literature to diminish the effects of vortex-induced vibration through references such as ASME STS or CICIND. However, these references were for large scale items such as chimneys and stacks. Application to the specific flow problem was not straightforward.
An analysis was undertaken to prove the effectiveness of the helical strakes by comparison with a plain cylinder in cross flow. Several strake geometry designs were trialled to determine the most effective arrangement.
The assessment was completed using Computational Fluid Dynamics (CFD), considering three-dimensional transient flow past the two configurations. Results of the analysis confirmed that the plain cylinder, under the specified design conditions, was subject to vortex shedding. Results were in close agreement with theoretical and experimental values.
The selected helical strakes proved to be an effective solution suppressing vortex induced vibration on the thermowell.
For this project, FEC took a physical problem with no obvious engineering outcome and delivered a final solution for the client. Activities included:
- ASME PTC 19.3 review for a range of designs
- Literature review
- Review of vibration collar options and frequency prediction
- Concept development and proof of design for a helical thermowell
- Detailed fabrication and machining drawings for the Inconel 800H thermowell assembly
- Final approved design suitable for implementation.
CFD removed the limitations associated with available standards. It provided a path to solve the problem, which didn’t involve extending the shutdown or re-work to the transfer line and compromising the newly cast refractory.